1. Field of the Invention
This invention relates to a rotary fluid displacement apparatus, and more particularly, to an improvement in a rotation preventing/thrust bearing device for an orbiting member fluid displacement apparatus.
2. Description of the Prior Art
There are several types of fluid apparatus which utilize an orbiting piston or fluid displacing member, such as a scroll type fluid displacement apparatus disclosed in U.S. Pat. No. 801,182 (Creux).
The scroll type fluid displacement apparatus disclosed in this U.S. patent includes two scrolls each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
Generally, in conventional scroll type fluid displacement apparatus, one scroll is fixed to a housing and the other scroll, which is the orbiting scroll, is eccentrically supported on a crank pin of a rotating shaft to cause the orbital motion. The scroll type fluid displacement apparatus also includes a rotation preventing device which prevents the rotation of the orbiting scroll to thereby maintain both scrolls in a predetermined angular relationship during operation of the apparatus.
Sealing along the line contacts of the above conventional scroll type apparatus must be maintained because the fluid pockets are defined by the line contacts between the two spiral elements and as the line contacts shift along the surface of the spiral elements, the fluid pockets change volume due to the orbital motion of the orbiting scroll. Since the orbiting scroll in such conventional scroll type apparatus is supported in a cantilever manner, an axial slant of the orbiting scroll occurs. Axial slant also occurs because the movement of the orbiting scroll is not rotary motion around the center of the orbiting scroll, but is orbiting motion caused by eccentric movement of a crank pin driven by the rotation of a drive shaft. Several problems result from the axial slant; such as, loss of sealing of the line contact and vibration of the apparatus during operation and noise caused by physical striking of the spiral elements.
One simple and direct solution to this problem is the use of a thrust bearing device for carrying the axial load. Thus, scroll type fluid displacement apparatus have been provided with rotation preventing and thrust bearing devices within their housing.
One recent attempt to improve rotation preventing and thrust bearing devices for scroll type fluid displacement apparatus is described in U.S. Pat. Nos. 4,160,629 (Hidden et al.) and 4,259,043 (Hidden et al.). The rotation preventing and thrust bearing devices in these U.S. patents are integral with one another. The rotation preventing/thrust bearing device described in these U.S. patents (see, e.g., FIG. 7 of Hidden et al. U.S. Pat. No. 4,259,043), comprises one set of indentations formed on the end surface of the circular plate of the orbiting scroll and a second set of indentations formed on an end surface of a fixed plate attached to the housing. A plurality of spheres are placed between facing indentations. However, the identations are formed directly on the end surface of orbiting scroll or the fixed plate. The production of this type of mechanism is therefore very intricate.
Referring to FIGS. 1, 2 and 3, which are illustrated in U.S. Pat. No. 4,492,543 (Iimori et al.), one solution to the above disadvantage will be described. FIG. 1 is a vertical section view of a part of a compressor and FIG. 2 is an exploded perspective view of a rotation preventing/thrust bearing device 500. Rotation preventing/thrust bearing device 500 surrounds a boss 527a of orbiting scroll 527 and includes an orbital portion, a fixed portion and bearings, such as a plurality of balls. The fixed portion includes (1) a first annular race 537 having one end surface fitted against the axial end surface of annular projection 512 of front end plate 511, and (2) a first ring 538 fitted against the other axial end surface of first race 537 to extend outwardly therefrom and cover the other axial end surface of first race 537. First race 537 and first ring 538 are attached to the axial end surface of annular projection 512 by pins 539. The orbital portion also includes (1) a second annular race 547, which has one end surface fitted against an axial end surface of circular plate 527b and (2) a second ring 548 fitted against the other axial end surface of second race 547 to extend outwardly therefrom and cover the other axial end surface of second race 547. A small clearance is maintained between the end surface of first ring 538 and the end surface of second ring 548. Second race 547 and second ring 548 are attached to the end surface of circular plate 527b by pins 549.
First ring 538 and second ring 548 each have a plurality of holes or pockets 538a and 548a in the axial direction, the number of holes or pockets in each ring 538, 548 being equal. Bearing elements, such as balls or spheres 550, are placed between facing generally aligned pairs of pockets 538a, 548a of first and second rings 538, 548, with the rings 538, 548 facing one another at a predetermined clearance.
Referring to FIG. 3, the operation of the rotation preventing/thrust bearing device 500 will be described. In FIG. 3, the center of second ring 548 is placed at the right side and the rotating direction of the drive shaft is clockwise as indicated by arrow "A". When orbiting scroll 527 is driven by the rotation of the drive shaft, the center of second ring 548 orbits about a circle of radius "R.sub.o " (together with orbiting scroll 527). However, a rotating force, i.e., moment, which is caused by the offset of the acting point of the reaction force of compression and the acting point of drive force, acts on orbiting scroll 527. This reaction force tends to rotate orbiting scroll 527 in a clockwise direction about center of second ring 548. But, as shown in FIG. 3, eighteen balls 550 are placed between the corresponding pockets 538a and 548a of rings 538 and 548. In the position shown in FIG. 3, the interaction between the nine balls 550 at the top of the rotation preventing/thrust bearing device and the edges of the pockets 538a and 548a prevents the rotation of orbiting scroll 527. The magnitude of the rotation preventing forces are shown as fc.sub.1 -fc.sub.5 in FIG. 3.
In the construction, as described above, the rotation preventing/thrust bearing device 500 is made up of a pair of races and a pair of rings, with each race and ring formed separately. Therefore, the parts of the rotation/thrust bearing device are easy to construct and the most suitable material for each part can be selected, for example, steel for each part. However, as shown in FIG. 4, since first and second races 537, 547 are fixedly attached to the axial end surface of annular projection 512 and the end surface of circular end plate 527b, respectively, each of the spheres 550 traces a circle onto first and second races 537, 547 during orbital motion of orbiting scroll 527. As a result, the surfaces of first and second races 537, 547 receiving the spheres tends to easily exfoliate at the circular trace over a lapse of time. Consequentially, the exfoliation extends radially inwardly and radially outwardly from the circular trace so that the life of the compressor is reduced.
On the other hand, referring to FIG. 5, when second race 547' is loosely disposed on the end surface of circular plate 527b' in a radial direction and is covered by the second ring with a slight axial clearance, second race 547' can freely rotate during orbital motion of the orbiting scroll. Therefore, as shown in FIG. 6, the spheres trace an annular area T onto second race 547'. Since the above relation between the second race 547' and the circular plate 527b' of the orbiting scroll is applicable to the first race and the front end plate, the spheres also trace an annular area onto the first race as well. Accordingly, the annular area of the first and second races are rolled by virtue of receiving a thrust force generated by gas pressure in the fluid pockets through spheres so that the first and second races 537', 547' warp in the opposite direction as shown in FIG. 7. Therefore, the races are required to be sufficiently thick to prevent the warp thereof. Thereby, weight of the races is increased so that weight of the compressor is also increased.